MODELING BIASES IN ESTIMATING PRODUCTION FROM COPEPOD COHORTS

Production (P) by copepod cohorts between sample times 0 and t is esti mated from cohort numbers (N-0 and N-t) and biomasses (B-0 and B-t) as P = (I + m/g)Delta B, where m = In(N-0/N-t), g = In(B-t/B-0), and Del ta B = B-t - B-0. This equation assumes exponential changes in biomass and numbers during the interval. Individual-based simulation models s how that variances in initial weights and growth rates do not bias est imates; only severe size (stage) dependence of growth rates introduces serious bias; extreme , size dependence of mortality rates produces m ore bias, mitigated by more frequent sampling; negative size dependenc e of both growth and mortality rates tends to cancel biases; and under estimation of production when individuals accumulate in nongrowing sta ges is mitigated by sampling when such individuals first appear. With these qualifications, the above equation is adequate and detailed mort ality and growth rates of individual stages are unnecessary when cohor ts are distinguishable.